Differential changes in steroid hormones before competition in bonobos and chimpanzees

Abstract

A large body of research has demonstrated that variation in competitive behavior across species and individuals is linked to variation in physiology. In particular, rapid changes in testosterone and cortisol during competition differ according to an individual's or species' psychological and behavioral responses to competition. This suggests that among pairs of species in which there are behavioral differences in competition, there should also be differences in the endocrine shifts surrounding competition. We tested this hypothesis by presenting humans' closest living relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), with a dyadic food competition and measuring their salivary testosterone and cortisol levels. Given that chimpanzees and bonobos differ markedly in their food-sharing behavior, we predicted that they would differ in their rapid endocrine shifts. We found that in both species, males showed an anticipatory decrease (relative to baseline) in steroids when placed with a partner in a situation in which the two individuals shared food, and an anticipatory increase when placed with a partner in a situation in which the dominant individual obtained more food. The species differed, however, in terms of which hormone was affected; in bonobo males the shifts occurred in cortisol, whereas in chimpanzee males the shifts occurred in testosterone. Thus, in anticipation of an identical competition, bonobo and chimpanzee males showed differential endocrine shifts, perhaps due to differences in perception of the situation, that is, viewing the event either as a stressor or a dominance contest. In turn, common selection pressures in human evolution may have acted on the psychology and the endocrinology of our competitive behavior.A cross species, including humans, males engaged in competition tend to show acute shifts in their levels of steroid hormones, such as testosterone and cortisol. These hormones change in a matter of minutes surrounding a competitive event, in anticipation of the competition and in response to its outcome (1, 2). In humans, men normally demonstrate an increase in cortisol before competition (3, 4). After the competition male winners tend to maintain their testosterone levels, whereas male losers' testosterone decreases (5, 6). In other animals, competing males show similar rapid changes in glucocorticoids and testosterone, since these hormones are thought to mediate energy allocation toward mating effort across species (7-10). Because competition for overt markers of status and mating opportunities is more relevant to males, these effects are less consistent in females (11)(12)(13)(14). Beyond these typical patterns, there is also high variability within and between species in the nature of the hormonal shifts surrounding competition that may be shaped by the psychology underlying competitive behavior.Two main psychological factors have been implicated in governing the endocrine changes surrounding competition within and between species: implicit power motive and coping styl...

Oxytocin reactivity during intergroup conflict in wild chimpanzees

“…Our results demonstrate an anticipatory increase in urinary oxytocin in a similar manner to the anticipatory testosterone increase found in intergroup conflicts (28) and intragroup competition (29) in chimpanzees. The observed high urinary oxytocin levels before border patrol initiation suggest that individuals may anticipate imminent intergroup conflict.…”

Decision making across social contexts: competition increases preferences for risk in chimpanzees and bonobos

“…In terms of aggression, chimpanzees show more linear dominance hierarchies and more severe (and sometimes lethal) aggression than do bonobos (Kano 1992;Watts & Mitani 2001;Boesch et al 2002;Muller et al 2009). Experimental comparisons further indicate that bonobos are more able to share food and tolerantly co-feed when than are chimpanzees Wobber et al 2010a, b). One prediction from this set of observational and experimental results is that chimpanzees might respond more strongly to the threat of competition than bonobos.…”

“…An analogous phenomenon occurs in these species' hormonal responses to conspecific competition. In particular, while both species show anticipatory changes in steroid hormones prior to interactions involving food, bonobos show these shifts in cortisol, whereas chimpanzees show similar shifts in testosterone levels (Wobber et al 2010a). That is, while the behavioural response to competition was similar, the neurohormonal mechanisms underpinning the responses varied across the two species.…”

“…One possible explanation is that the current study involved a human competitor, not conspecifics. Observations of these species in the wild obviously involve conspecific interactions, as do several experimental studies involving conspecific partners Wobber et al 2010a, b). Importantly, these types of observations and experiments cannot equate the behaviour of social partners across species, and are indeed not designed to do so, as their goal is to understand naturalistic social interactions.…”

Investigating the relationship between welfare and rearing young in captive chimpanzees (Pan troglodytes)

“…Social feeding has been shown to be an arousing situation for chimpanzees both behaviourally and physiologically (Wobber et al, 2010). At the same time every day, social groups are verbally called inside an attached building through two shift doors into a series of adjacent rooms in which they receive food.…”